1. Field of the Invention
The present invention relates to a vehicle motion control apparatus.
2. Description of the Related Art
Conventionally, there has been widely known a vehicle motion control apparatus that performs a vehicle stabilization control in order to maintain running stability of the vehicle during turning. Specifically, a vehicle motion control apparatus of such a type determines that the vehicle is in a state of under-steer, for example, when the difference between a vehicle yaw rate (hereinafter referred to as “actual yaw rate”) obtained from a yaw rate sensor (or a lateral acceleration sensor) and a yaw rate (hereinafter referred to as “turning angle yaw rate”) calculated from steering angle (turning angle of the steerable wheels) obtained from a steering angle sensor (and vehicle body speed, specifications of the vehicle, etc.,) exceeds a predetermined threshold value.
In case where the apparatus of such a type determines that the vehicle is in an under-steer, in general, it imparts a predetermined braking force, by means of brake hydraulic pressure, to the rear wheel located on the inner side of a turning locus in order to generate a yawing moment (under-steer suppressing moment) in the vehicle in a direction same as the vehicle's yawing direction. With this operation, the under-steer suppression control is executed, whereby a yaw rate deviation is controlled to be not more than the threshold value.
This apparatus is composed of an integrated unit made integrally of a hydraulic unit having mounted thereto plural solenoid valves and plural hydraulic devices such as a hydraulic pump that are required for controlling braking force exerted on wheels, and an electronic control apparatus (ECU) for controlling plural hydraulic devices; and various sensors such as the aforesaid yaw rate sensor, steering angle sensor, and the like, which are separate from the integrated unit and connected to the integrated unit via harnesses, connectors or the like. In this case, the integrated unit executes the aforesaid vehicle stabilization control in receipt of signals from various sensors via a so-called CAN communication.
In recent years, a technique has been developed for incorporating the yaw rate sensor (or lateral acceleration sensor) into the integrated unit (e.g., see the following Patent Reference 1). According to this technique, harnesses and connectors can be omitted, and further, electronic parts such as a CPU, CAN driver or the like in the yaw rate sensor required for the CAN communication can also be omitted, so that the manufacturing cost for the whole apparatus can be reduced.
[Patent Reference]
Japanese National Publication No. 2004-506572
In the integrated unit, a vibration is generated by the operation of the hydraulic devices such as hydraulic pump, solenoid valves or the like mounted to the integrated unit. In addition, the integrated unit is indirectly fixed to the vehicle body via a mount, so that the vibration at the side of the vehicle body received from the road surface can be amplified by the resonance and transmitted to the integrated unit.
Accordingly, when the yaw rate sensor is incorporated in the integrated unit, various vibrations given to the integrated unit can directly be transmitted to the yaw rate sensor, and hence, the vibration imparted to the yaw rate sensor increases. When the vibration imparted to the yaw rate sensor increases, the value of the actual yaw rate (see bold broken line) is liable to be fluctuating (vibration noise is liable to be superimposed) with respect to the value of the true yaw rate (see two-dot-chain line) as shown in FIG. 10. As a result, the value of the yaw rate deviation is also liable to be fluctuating, so that there may be a possibility that the vehicle stabilization control is not appropriately executed.
From the above, the apparatus disclosed in the aforesaid Reference calculates, instead of the actual yaw rate itself obtained from the yaw rate sensor incorporated in the integrated unit, the yaw rate deviation based upon the value obtained by providing a low-pass filter process to the actual yaw rate, and starts the vehicle stabilization control (e.g., under-steer suppression control) when the yaw rate deviation exceeds the threshold value.
However, the following problem arises by this configuration. It is supposed that the true yaw rate deviation, which is obtained by subtracting the true yaw rate from the turning angle yaw rate, exceeds the threshold value at time t2 as shown in FIG. 10, i.e., that the vehicle stabilization control (specifically, under-steer suppression control) should be started at time t2.
In general, when a low-pass filter process is provided to the fluctuating signal (value), the value to which the low-pass filter has been provided fluctuates with the delay according to the time constant of the low-pass filter from the value to which the low-pass filter has not yet been provided. Accordingly, as shown in FIG. 10, the value, which is obtained by providing the low-pass filter to the actual yaw rate (bold broken line) (actual-yaw-rate-after-low-pass-filter-process, see thin broken line), also fluctuates with the delay from the actual yaw rate.
This acts in the direction in which the value of the yaw rate deviation (i.e., the value obtained by subtracting the actual-yaw-rate-after-low-pass-filter-process from the turning angle yaw rate) increases under the condition where the true yaw rate deviation increases, i.e., under the condition where the point when the vehicle stabilization control should be started reaches.
This allows the yaw rate deviation to exceed the threshold value at time t1 that is before time t2, regardless of the fact that the true yaw rate deviation does not exceed the threshold value before time t2. As a result, there arises a problem that a malfunction may be caused in which the under-steer suppression control is started earlier from time t1.